Ratio shifting in a vehicle transmission is generally initiated in response to the achievement of predefined load conditions represented by predefined combinations of vehicle speed and engine throttle position. In electronically controlled transmissions, data corresponding to the predefined combinations of vehicle speed and engine throttle position are stored in a look-up table or similar data structure. Measured values of vehicle speed and engine throttle position are compared to the stored data to determine the desired speed ratio, and a shift is initiated if the desired ratio is different from the actual ratio. If the desired ratio is higher than the actual ratio, an upshift is initiated; if the desired ratio is lower than the actual ratio, a downshift is initiated. In practice, separate data is maintained for upshifts and downshifts in order to provide a degree of hysteresis which avoids hunting and unnecessary heating of the transmission under steady state conditions.
The above described technique is graphically depicted for a four-speed transmission in FIG. 1c, where upshift data is represented by the solid lines 1-2, 2-3 and 3-4, and downshift data is represented by the broken lines 2-1, 3-2 and 4-3. The pairs of upshift and downshift lines divide the ranges of vehicle speed and engine throttle position into four regions corresponding to the four transmission ratios 1st, 2nd, 3rd and 4th. The separation between the respective pairs of upshift and downshift lines (3-2 and 2-3, for example) provides the hysteresis referred to above.
The transmission controller repeatedly compares measured values of vehicle speed and engine throttle position with ratio-dependent data from the table to determine the desired ratio. If the actual ratio is 1st, the measured vehicle speed and engine throttle position values are compared with the 1-2 upshift line; if the actual ratio is 2nd, the measured values are compared with the 2-1 downshift line and the 2-3 upshift line; if the actual ratio is 3rd, the measured values are compared with the 3-2 downshift line and the 3-4 upshift line; and if the actual ratio is 4th, the measured values are compared with the 4-3 downshift line.
Shifting the transmission from one speed ratio to another is achieved by engaging and disengaging various fluid operated torque transmitting devices (referred to herein as clutches) within the transmission. During the course of each such shift, a certain amount of friction-related heat is generated and absorbed by the torque transmitting devices involved in the shift. The heat is slowly dissipated into the transmission fluid and housing, and the various clutches are sized to withstand the heat of shifting which would occur in normal driving conditions. Under unusual or abusive driving conditions, however, the frequency of shifting may significantly exceed the normal expectation, thereby thermally stressing the transmission clutches.